1. The Field of the Invention
The present invention relates generally to x-ray tubes. More particularly, the present invention relates to cathode assemblies.
2. Related Technology
The x-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. Such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
An x-ray tube typically includes a vacuum enclosure, a cathode assembly, and an anode assembly. The vacuum enclosure may be composed of metals such as copper, glass, ceramic, or a combination thereof, and is typically disposed within an outer housing. At least a portion of the outer housing may be covered with a shielding layer composed of, for example, lead or a similar x-ray attenuating material for preventing the escape of x-rays produced within the vacuum enclosure. In addition, a cooling medium, such as a dielectric oil or similar coolant, can be disposed in the volume existing between the outer housing and the vacuum enclosure in order to dissipate heat from the surface of the vacuum enclosure.
The cathode assembly of the x-ray tube generally consists of a metallic cathode head and a source of highly energized electrons. The anode assembly of the x-ray tube includes a target surface, which is generally manufactured from a refractory metal such as tungsten and is oriented to receive electrons emitted by the cathode assembly.
During operation of the x-ray tube, the cathode may be charged with a heating current that causes electrons to “boil” off the electron source by the process of thermionic emission. An electric potential can be applied between the cathode and the anode in order to accelerate electrons emitted by the electron source toward the target surface of the anode assembly. X-rays are generated when the highly accelerated electrons strike the target. Some of the x-rays that are produced by these processes ultimately exit the x-ray tube through a window and interact with a patient, a material sample, or another object.
It is generally desirable to maximize the focusing of the electron stream on the anode surface in order to produce a tightly collimated x-ray beam. It is well understood that the quality of diagnostic images additionally depends on the pattern, or focal spot, created by the emitted beam of electrons from the cathode onto the target surface of the target anode. In general, a smaller focal spot produces a more highly focused or collimated beam of x-rays, which in turn produces better quality x-ray images.
The characteristics of the focal spot may be affected by the configuration of the components of the cathode assembly. However, many cathode assemblies are configured in such a way that they impair the effectiveness with which the focal spot can be defined and/or maintained